Comfort of shoes can be crucial to the persons wearing them, both in respect of the persons' general well-being, where shoes are worn often every day and for many hours, but also in connection with the performance of a person when using the shoes.
Typically, shoes are being made in series having different kinds of properties, where one shoe series has a sole with a specific flexibility and another shoe series has another flexibility. The properties of each series are then determined based on different tests being made on models of a shoe and based on feedback from users of prior series.
All users have feet, which are uniquely shaped, and similarly, the dynamics of a foot is unique—for this reason, it will be impossible or at least difficult for a user to find a shoe, which has the optimal properties. Since the user will have to choose between series of produced shoes, the user will have to compromise and find the best shoe series available—not the optimal shoe.
Furthermore, making specific measurements at the shoe store by looking at the feet of the user, making a short usage test (e.g. on a running belt or similar) and further by looking at the wear characteristics of the older pair of shoes is typically the full extent of identifying the best shoe for the user.
A further problem by having series of shoes is that both shoes in a pair of shoes are made having similar properties being basically mirrored to fit respectively a left and a right foot, but when it comes to the feet of a user then the dynamics and shape can be quite different, whereby the shoes in a pair of shoes will be more suitable to one foot than the other.